Chronic Stress Impairs the Structure and Function of Astrocyte Networks in an Animal Model of Depression

Neuroglial Advances: New Roles for Established Players

Neuroglial cells perform numerous physiological functions and contribute to the pathogenesis of all diseases of the nervous system. Neuroglial neuroprotection defines the resilience of the nervous tissue to exo- and endogenous pathological challenges, while neuroglial defence determines the progression and outcome of neurological disorders. IN this paper, we overview previously unknown but recently discovered roles of various types of neuroglial cells in diverse physiological and pathological processes. First, we describe the role of ependymal glia in the regulation of cerebrospinal fluid flow from the spinal cord to peripheral tissues through the spinal nerves. This newly discovered pathway provides a highway for the CNS-body volume transmission. Next, we present the mechanism by which astrocytes control migration and differentiation of oligodendrocyte precursor cells (OPCs). In pre- and early postnatal CNS, OPCs migrate using vasculature (which is yet free from glia limitans perivascularis) as a pathfinder. Newly forming astrocytic perivascular endfeet signal (through semaphorin-plexin cascade) to OPCs that detach from the vessels and start to differentiate into myelinating oligodendrocytes. We continue the astrocyte theme by demonstrating the neuroprotective role of APOE-laden astrocytic extracellular vesicles in neuromyelitis optica. Next, we explore the link between astrocytic morphology and stress-induced depression. We discuss the critical role of astrocytic ezrin, the cytosolic linker defining terminal astrocyte arborisation and resilience to stress: overexpression of ezrin in prefrontal cortical astrocytes makes mice resistant to stress, whereas ezrin knockdown increases animals vulnerability to stress. Subsequently, we highlight the pathophysiological role of oligodendroglial lineage in schizophrenia by describing novel hypertrophied OPCs in the post-mortem patient's tissue and in a mouse model with OPCs overexpressing alternative splice variant DISC1-Δ3. These DISC1-Δ3-OPCs demonstrated overactivated Wnt/β-catenin signalling pathway and were sufficient to trigger pathological behaviours. Finally, we deliberate on the pathological role of astrocytic and microglial connexin 43 hemichannels in Alzheimer's disease and present a new formula of Cx43 hemichannel inhibitor with increased blood-brain barrier penetration and brain retention.

Astrocytic CREB in Nucleus Accumbens Promotes Susceptibility to Chronic Stress

BACKGROUND

Increasing evidence implicates astrocytes in stress and depression in both rodent models and human major depressive disorder. Despite this, little is known about the transcriptional responses to stress of astrocytes within the nucleus accumbens (NAc), a key brain reward region, and their influence on behavioral outcomes.

METHODS

We used whole-cell sorting, RNA sequencing, and bioinformatic analyses to investigate the NAc astrocyte transcriptome in male mice in response to chronic social defeat stress (CSDS). Immunohistochemistry was used to determine stress-induced changes in astrocytic CREB (cAMP response element binding protein) within the NAc. Finally, astrocytic regulation of depression-like behavior was investigated using viral-mediated manipulation of CREB in combination with CSDS.

RESULTS

We found a robust transcriptional response in NAc astrocytes to CSDS in stressed mice, with changes seen in both stress-susceptible and stress-resilient animals. Bioinformatic analysis revealed CREB, a transcription factor widely studied in neurons, as one of the top-predicted upstream regulators of the NAc astrocyte transcriptome, with opposite activation states implicated in resilient versus susceptible mice. This bioinformatic deduction was confirmed at the protein level with immunohistochemistry. Moreover, NAc astrocyte morphological complexity correlated with CREB activation and was reduced selectively in astrocytes of resilient mice. Viral overexpression of CREB selectively in NAc astrocytes promoted susceptibility to chronic stress.

CONCLUSIONS

Together, our data demonstrate that the astrocyte transcriptome responds robustly to CSDS and that transcriptional regulation in astrocytes contributes to depressive-like behaviors. A better understanding of transcriptional regulation in astrocytes may reveal unknown molecular mechanisms underlying neuropsychiatric disorders.

High urea promotes mitochondrial fission and functional impairments in astrocytes inducing anxiety-like behavior in chronic kidney disease mice

High urea can induce depression and anxiety. Activation of astrocytes is closely associated with psychiatric disorders. However, the pathological mechanism of whether high urea affects astrocyte structure and function to induce anxiety-like behaviors remain unclear. We established a high-urea chronic kidney disease (CKD) mouse model and found that these mice exhibited elevated levels of anxiety through behavioral experiments. Immunofluorescence and transmission electron microscopy studies of astrocytes revealed a decrease in density and branching of mPFC astrocytes. Additionally, we observed a significant reduction in ATP and BDNF levels in the mPFC and primary astrocytes of CKD mice induced by high urea. Analysis of gene expression differences in astrocytes between WT and high-urea mice indicated alterations in mitochondrial dynamics-related signaling pathways in astrocytes. We established a high-urea primary astrocyte model to assess mitochondrial function and levels of fusion and fission proteins. Treatment of primary astrocytes with high urea led to mitochondrial fragmentation and downregulation of Mfn2 expression. These results suggested that high urea downregulates Mfn2 expression in mPFC astrocytes, induced mitochondrial fusion-fission abnormalities, disrupted astrocyte energy metabolism, and promoted high-urea-related anxiety. Mfn2 may represent a potential therapeutic target for high-urea-related anxiety.

Stress-induced dysfunction of neurovascular astrocytes in the prefrontal cortex contributes to sex-dependent deficits in cognition and behavior

Astrocytes form an integral component of the neurovascular unit, ensheathing brain blood vessels with endfeet high in aquaporin-4 (AQP4) expression. These AQP4-rich endfeet facilitate interaction between the vascular endothelium, astrocytes, and neurons, and help stabilize vascular morphology. Studies using preclinical models of psychological stress and post-mortem tissue from patients with major depressive disorder (MDD) have reported reductions in AQP4, loss of astrocytic structures, and vascular impairment in the prefrontal cortex (PFC). Though compelling, the role of AQP4 in mediating stress-induced alterations in neurovascular function and behavior remains unclear. Here, we address this, alongside potential sex differences in chronic unpredictable stress (CUS) effects on astrocyte phenotype, blood-brain barrier integrity, and behavior. CUS led to more pronounced shifts in stress-coping behavior and working memory deficits in male- as compared to female mice. Following behavioral testing, astrocytes from the frontal cortex were isolated for gene expression analyses. We found that CUS increased transcripts associated with blood vessel maintenance in males, but either had no effect on- or decreased- these transcripts in females. Furthermore, CUS caused a reduction in vascular-localized AQP4 and elevated extravasation of a small fluorescent reporter (Dextran) in the PFC in males but not females. Studies showed that knockdown of AQP4 in the PFC is sufficient to disrupt astrocyte phenotype and increase behavioral susceptibility to a sub-chronic stressor in males yet has little effect on stress susceptibility in females. Our findings provide evidence that sex-specific alterations in astrocyte phenotype and neurovascular integrity in the PFC contribute to cognitive-behavioral consequences following stress.

Chronic stress-induced neuroplasticity in the prefrontal cortex: Structural, functional, and molecular mechanisms from development to aging

Chronic stress profoundly affects the structure and function of the prefrontal cortex (PFC), a brain region critical for executive functions and emotional regulation. This review synthesizes current knowledge on stress-induced PFC plasticity, encompassing structural, functional, and molecular changes. We examine how chronic stress leads to dendritic atrophy, spine loss, and alterations in neuronal connectivity within the PFC, particularly affecting the medial PFC. These structural changes are accompanied by disruptions in neurotransmitter systems, most notably glutamatergic and GABAergic signaling, and alterations in synaptic plasticity mechanisms. At the molecular level, we discuss the intricate interplay between stress hormones, neurotrophic factors, and epigenetic modifications that underlie these changes. The review highlights the significant behavioral and cognitive consequences of stress-induced PFC plasticity, including impairments in working memory, decision-making, and emotional regulation, which may contribute to the development of stress-related psychiatric disorders. We also explore individual differences in stress susceptibility, focusing on sex-specific effects and age-dependent variations in stress responses. The role of estrogens in conferring stress resilience in females and the unique vulnerabilities of the developing and aging PFC are discussed. Finally, we consider potential pharmacological and non-pharmacological interventions that may mitigate or reverse stress-induced changes in the PFC. The review concludes by identifying key areas for future research, including the need for more studies on the reversibility of stress effects and the potential of emerging technologies in unraveling the complexities of PFC plasticity. This comprehensive overview underscores the critical importance of understanding stress-induced PFC plasticity for developing more effective strategies to prevent and treat stress-related mental health disorders.

An Astroglial Basis of Major Depressive Disorder: Molecular, Cellular, and Circuit Features

Major depressive disorder is a common psychiatric disorder and a leading cause of disability worldwide. Astrocytes play a role in the maintenance of the function of the central nervous system, both physiologically and pathologically. Accumulated evidence indicates that the astrocyte is an important contributor to the pathophysiology of major depressive disorder including blood-brain barrier integrity, gap junctions, gliotransmission, glutamate homeostasis, and energy metabolism. Here, we comprehensively summarize an astroglial basis for major depressive disorder based on molecular, cellular, and circuit properties, suggesting that astrocytes appear to be highly sensitive to stress and are likely to be uniquely positioned to integrate peripheral and central stress responses.

Revealing and Characterizing Astrocytic Atrophy in an Animal Model of Depression

Astrocyte atrophy is the main histopathological hallmark of major depressive disorder (MDD) in humans and in animal models of depression. In particular, depression and depressive-like behaviors are associated with a substantial decrease in size and complexity of astrocytes in the prefrontal cortex. This results in the reduced homeostatic support of synaptic transmission, which arguably translates into abnormal activity of neuronal networks and pathological changes in mood and behavior. Treatment of experimental animals with antidepressants as well as with acupuncture in the specific acupoints alleviates depressive-like behaviors and rescues astrocytic atrophy. Here, we describe the methodology for inducing and monitoring the depressive-like behaviors in mice using chronic unpredictable mild stress (CUMS), with subsequent in-depth analysis of astrocytic morphology using confocal microscopy in conjunction with astrocyte-specific labeling by virally transfected genetically encoded fluorescent probe mCherry and intracellular injection of Lucifer yellow. We also describe immunocytochemical visualization of astrocyte-specific cytoskeletal linker ezrin, which is involved in controlling perisynaptic astrocytic leaflets.

General pathophysiology of neuroglia

Neuroglia in the CNS, represented by astroglia, oligodendroglia, and microglia, are responsible for the homeostatic support and protection of the nervous tissue. Neuroglia are intimately involved in the pathogenesis of all neurologic diseases, and neuroglial changes to a large extent define the progression of these diseases and their neurologic outcome. In contrast to neurons, neuroglia are capable of mounting an evolutionary conserved response to pathology known as reactive gliosis. Reactive gliosis is initially protective and allostatic, and it is aimed at preserving the nervous tissue function and integrity. However, in many diseases, neuroglial cells undergo atrophy and functional asthenia, contributing to nervous tissue damage.

Ginsenoside Rg1: A Neuroprotective Natural Dammarane-Type Triterpenoid Saponin With Anti-Depressive Properties

BACKGROUND

Depression, a widespread mental disorder, presents significant risks to both physical and mental health due to its high rates of recurrence and suicide. Currently, single-target antidepressants typically alleviate depressive symptoms or delay the progression of depression rather than cure it. Ginsenoside Rg1 is one of the main ginsenosides found in Panax ginseng roots. It improves depressive symptoms through various mechanisms, suggesting its potential as a treatment for depression.

MATERIALS AND METHODS

We evaluated preclinical studies to comprehensively discuss the antidepressant mechanism of ginsenoside Rg1 and review its toxicity and medicinal value. Additionally, pharmacological network and molecular docking analyses were performed to further validate the antidepressant effects of ginsenoside Rg1.

RESULTS

The antidepressant mechanism of ginsenoside Rg1 may involve various pharmacological mechanisms and pathways, such as inhibiting neuroinflammation and over-activation of microglia, preserving nerve synapse structure, promoting neurogenesis, regulating monoamine neurotransmitter levels, inhibiting hyperfunction of the hypothalamic-pituitary-adrenal axis, and combatting antioxidative stress. Moreover, ginsenoside Rg1 preserves astrocyte gap junction function by regulating connexin43 protein biosynthesis and degradation, contributing to its antidepressant effect. Pharmacological network and molecular docking studies identified five targets (AKT1, STAT3, EGFR, PPARG, and HSP90AA1) as potential molecular regulatory sites of ginsenoside Rg1.

CONCLUSIONS

Ginsenoside Rg1 may exert its antidepressant effects via various pharmacological mechanisms. In addition, multicenter clinical case-control and molecular targeted studies are required to confirm both the clinical efficacy of ginsenoside Rg1 and its potential direct targets.

Neuroglia in cognitive reserve

The concept of cognitive reserve was born to account for the disjunction between the objective extent of brain damage in pathology and its clinical and intellectual outcome. The cognitive reserve comprises structural (brain reserve) and functional (brain maintenance, resilience, compensation) aspects of the nervous tissue reflecting exposome-driven life-long plasticity, which defines the ability of the brain to withstand aging and pathology. The mechanistic background of this concept was primarily focused on adaptive changes in neurones and neuronal networks. We present arguments favoring the more inclusive view, positing that neuroglia are fundamental for defining the cognitive reserve through homeostatic, neuroprotective, and neurodegenerative mechanisms. Neuroglia are critical for the life-long shaping of synaptically connected neuronal circuits as well as the brain connectome thus defining cognitive reserve. Neuroglial homeostatic and protective physiological responses define brain maintenance and resilience, while neuroglia regenerative capabilities are critical for brain compensation in pathology. Targeting neuroglia may represent an untrodden path for prolonging cognitive longevity.

Neuroinflammation and major depressive disorder: astrocytes at the crossroads

Major depressive disorder is a complex and multifactorial condition, increasingly linked to neuroinflammation and astrocytic dysfunction. Astrocytes, along with other glial cells, beyond their classic functions in maintaining brain homeostasis, play a crucial role in regulating neuroinflammation and neuroplasticity, key processes in the pathophysiology of depression. This mini-review explores the involvement of astrocytes in depression emphasizing their mediation in neuroinflammation processes, the impact of astrocytic dysfunction on neuroplasticity, and the effect of some antidepressants on astrocyte reactivity. Recent evidence suggests that targeting astrocyte-related signaling pathways, particularly the balance between different astrocytic phenotypes, could offer promising evidence for therapeutic strategies for affective disorders. Therefore, a deeper understanding of astrocyte biology may open the way to innovative treatments aimed at mitigating depressive symptoms by impacting both neuroinflammation and imbalances in neuroplasticity.

Inflammation as a shared mechanism of chronic stress related disorders with potential novel therapeutic targets

Stress is a subjective experience that varies across individuals depending on their sensitivity, resilience, and length of exposure to stressors. Stress may be categorised as acute (positive stress) or chronic (negative stress). Acute stress is advantageous for the human body, but chronic stress results in changes in cardiovascular, neuroendocrine, autonomic, and immunological functions, eventually causing different illnesses. The specific process relating stress to chronic stress associated diseases is still a topic of continuing debate. Inflammation has been recognised as a new and fascinating physiological mechanism that connects chronic stress and its associated illnesses. This article explored the relationships between chronic stress, inflammation, and chronic illnesses, including depression, cancer, and cardiovascular disease. This article also emphasises on various possible therapeutic targets for the management of chronic stress related illnesses by targeting inflammation, namely lipoxins and alpha7 nicotinic receptors. These therapeutic targets may be useful in developing new and safe therapies for the management of chronic stress related dysfunctions.

Dynamic changes of media prefrontal cortex astrocytic activity in response to negative stimuli in male mice

Astrocytes play significant roles in regulating the central stress response. Chronic stress impairs the structure and function of astrocytes in many brain regions such as media prefrontal cortex (mPFC) in multiple neuropsychiatric conditions, but the astrocytic dynamics on the timescale of behavior remains unclear. Here, we recorded mPFC astrocytic activity in freely behaving mice and found that astrocytes are activated immediately by different aversive stimuli. Astrocyte specific GCaMP6s calcium indicator were virally expressed in mPFC astrocytes and fiber photometry experiments revealed that astrocytes are activated by tail-restraint (TRT), foot shock (FS), open arm exploration, stressor of height, predator odor and social defeat (SD) stress. ΔF/F analyses demonstrated that an unpredictable stimulus such as elevated platform stress (EPS) at the initial encounter induced the most intense and rapid changes in astrocytic calcium activity, while a predictable 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) stimulus resulted in the weakest response with a longer peak latency. In TRT, FS or SD test, a somatic stimulus led to higher average calcium activity level and faster average peak latency in repeated trails. Similar to TMT stimulus, astrocytic calcium activity in elevated plus maze (EPM) test exhibited a smaller average change in amplitude and the longest peak latency during open arm exploration. Moreover, astrocytic calcium activity exhibited different changes across behavioral states in SD tests. Our findings show that mPFC astrocytes exhibit distinct patterns of calcium activity in response to various negative stimuli, indicating that the dynamic activity of astrocytes may reflect the stress-related behavioral state under different stimulus conditions.

The multiple roles of chronic stress and glucocorticoids in Alzheimer's disease pathogenesis

Chronic stress and the accompanying long-term elevation of glucocorticoids (GCs), the stress hormones of the body, increase the risk and accelerate the progression of Alzheimer's disease (AD). Signatures of AD include intracellular tau (MAPT) tangles, extracellular amyloid β (Aβ) plaques, and neuroinflammation. A growing body of work indicates that stress and GCs initiate cellular processes underlying these pathologies through dysregulation of protein homeostasis and trafficking, mitochondrial bioenergetics, and response to damage-associated stimuli. In this review, we integrate findings from mechanistic studies in rodent and cellular models, wherein defined chronic stress protocols or GC administration have been shown to elicit AD-related pathology. We specifically discuss the effects of chronic stress and GCs on tau pathogenesis, including hyperphosphorylation, aggregation, and spreading, amyloid precursor protein (APP) processing and trafficking culminating in Aβ production, immune priming by proinflammatory cytokines and disease-associated molecular patterns, and alterations to glial cell and blood-brain barrier (BBB) function.

Novel insight into astrocyte-mediated gliotransmission modulates the synaptic plasticity in major depressive disorder

Major depressive disorder (MDD) is a form of glial cell-based synaptic dysfunction disease in which glial cells interact closely with neuronal synapses and perform synaptic information processing. Glial cells, particularly astrocytes, are active components of the brain and are responsible for synaptic activity through the release gliotransmitters. A reduced density of astrocytes and astrocyte dysfunction have both been identified the brains of patients with MDD. Furthermore, gliotransmission, i.e., active information transfer mediated by gliotransmitters between astrocytes and neurons, is thought to be involved in the pathogenesis of MDD. However, the mechanism by which astrocyte-mediated gliotransmission contributes to depression remains unknown. This review therefore summarizes the alterations in astrocytes in MDD, including astrocyte marker, connexin 43 (Cx43) expression, Cx43 gap junctions, and Cx43 hemichannels, and describes the regulatory mechanisms of astrocytes involved in synaptic plasticity. Additionally, we investigate the mechanisms acting of the glutamatergic, gamma-aminobutyric acidergic, and purinergic systems that modulate synaptic function and the antidepressant mechanisms of the related receptor antagonists. Further, we summarize the roles of glutamate, gamma-aminobutyric acid, d-serine, and adenosine triphosphate in depression, providing a basis for the identification of diagnostic and therapeutic targets for MDD.

Sex-dependent astrocyte reactivity: Unveiling chronic stress-induced morphological changes across multiple brain regions

Chronic stress is a major precursor to various neuropsychiatric disorders and is linked with increased inflammation in the brain. However, the bidirectional association between inflammation and chronic stress has yet to be fully understood. Astrocytes are one of the key inflammatory regulators in the brain, and the morphological change in reactive astrocytes serves as an important indicator of inflammation. In this study, we evaluated the sex-specific astrocyte response to chronic stress or systemic inflammation in key brain regions associated with mood disorders. We conducted the unpredictable chronic mild stress (UCMS) paradigm to model chronic stress, or lipopolysaccharide (LPS) injection to model systemic inflammation. To evaluate stress-induced morphological changes in astrocyte complexity, we measured GFAP fluorescent intensity for astrocyte expression, branch bifurcation by quantifying branch points and terminal points, branch arborization by conducting Sholl analysis, and calculated the ramification index. Our analysis indicated that chronic stress-induced morphological changes in astrocytes in all brain regions investigated. The effects of chronic stress were region and sex specific. Notably, females had greater stress or inflammation-induced astrocyte activation in the hypothalamus (HYPO), CA1, CA3, and amygdala (AMY) than males. These findings indicate that chronic stress induces astrocyte activation that may drive sex and region-specific effects in females, potentially contributing to sex-dependent mechanisms of disease.

Astrocyte-Mediated Neuroinflammation in Neurological Conditions

Astrocytes are one of the key glial types of the central nervous system (CNS), accounting for over 20% of total glial cells in the brain. Extensive evidence has established their indispensable functions in the maintenance of CNS homeostasis, as well as their broad involvement in neurological conditions. In particular, astrocytes can participate in various neuroinflammatory processes, e.g., releasing a repertoire of cytokines and chemokines or specific neurotrophic factors, which result in both beneficial and detrimental effects. It has become increasingly clear that such astrocyte-mediated neuroinflammation, together with its complex crosstalk with other glial cells or immune cells, designates neuronal survival and the functional integrity of neurocircuits, thus critically contributing to disease onset and progression. In this review, we focus on the current knowledge of the neuroinflammatory responses of astrocytes, summarizing their common features in neurological conditions. Moreover, we highlight several vital questions for future research that promise novel insights into diagnostic or therapeutic strategies against those debilitating CNS diseases.

Running exercise improves astrocyte loss, morphological complexity and astrocyte-contacted synapses in the hippocampus of CUS-induced depression model mice

Although the antidepressant effects of running exercise have been widely reported, further research is still needed to determine the structural bases for these effects. Astrocyte processes physically contact many synapses and directly regulate the numbers of synapses, but it remains unclear whether running exercise can modulate astrocyte morphological complexity and astrocyte-contacted synapses in the hippocampus of the mice with depressive-like behavior. Male C57BL/6 J mice underwent four weeks of running exercise after four weeks of chronic unpredictable stress (CUS). The sucrose preference test (SPT), tail suspension test (TST) and forced swim test (FST) were used to assess anhedonia in mice. Western blotting was used to measure the expression of astrocyte- and synapse-related proteins. Immunofluorescence and 3D reconstruction were used to quantify the density and morphology of astrocytes, and astrocyte-contacted synapses in each hippocampal subregion. Four weeks of running exercise alleviated depressive-like symptoms in mice. The expression of astrocyte- and synapse-related proteins in the hippocampus; astrocyte process lengths, process numbers, and dendritic arborization; and the number of astrocyte-contacted PSD95 positive synapses in the CA2-3 and DG regions were significantly decreased in the mice with depressive-like behavior, and running exercise successfully reserved these changes. Running exercise improved the decreases in astrocyte morphological complexity and astrocyte-contacted PSD95 positive synapses in the CA2-3 and DG regions of the mice with depressive-like behavior, suggesting that the physical interactions between astrocytes and synapses can be increased by running exercise, which might be an important structural basis for the antidepressant effects of running exercise.

How astrocytic chloride modulates brain states

The way the central nervous system (CNS) responds to diverse stimuli is contingent upon the specific brain state of the individual, including sleep and wakefulness. Despite the wealth of readout parameters and data delineating the brain states, the primary mechanisms are yet to be identified. Here we highlight the role of astrocytes, with a specific emphasis on chloride (Cl-) homeostasis as a modulator of brain states. Neuronal activity is regulated by the concentration of ions that determine excitability. Astrocytes, as the CNS homeostatic cells, are recognised for their proficiency in maintaining dynamic homeostasis of ions, known as ionostasis. Nevertheless, the contribution of astrocyte-driven ionostasis to the genesis of brain states or their response to sleep-inducing pharmacological agents has been overlooked. Our objective is to underscore the significance of astrocytic Cl- homeostasis, elucidating how it may underlie the modulation of brain states. We endeavour to contribute to a comprehensive understanding of the interplay between astrocytes and brain states.

Stress-induced dysfunction of neurovascular astrocytes contributes to sex-specific behavioral deficits

Astrocytes form an integral component of the neurovascular unit, ensheathing brain blood vessels with projections high in aquaporin-4 (AQP4) expression. These AQP4-rich projections facilitate interaction between the vascular endothelium, astrocytes, and neurons, and help stabilize vascular morphology. Studies using preclinical models of psychological stress and post-mortem tissue from patients with major depressive disorder (MDD) have reported reductions in AQP4, loss of astrocytic structures, and vascular impairment in the prefrontal cortex (PFC). Though compelling, the role of AQP4 in mediating stress-induced alterations in blood vessel function and behavior remains unclear. Here, we address this, alongside potential sex differences in chronic unpredictable stress (CUS) effects on astrocyte phenotype, blood-brain barrier integrity, and behavior. CUS led to pronounced shifts in stress-coping behavior and working memory deficits in male -but not female- mice. Following behavioral testing, astrocytes from the frontal cortex were isolated for gene expression analyses. We found that CUS increased various transcripts associated with blood vessel maintenance in astrocytes from males, but either had no effect on- or decreased- these genes in females. Furthermore, CUS caused a reduction in vascular-localized AQP4 and elevated extravasation of a small molecule fluorescent reporter (Dextran) in the PFC in males but not females. Studies showed that knockdown of AQP4 in the PFC in males is sufficient to disrupt astrocyte phenotype and increase behavioral susceptibility to a sub-chronic stressor. Collectively, these findings provide initial evidence that sex-specific alterations in astrocyte phenotype and neurovascular integrity in the PFC contribute to behavioral and cognitive consequences following chronic stress.

Not Just a Mood Disorder─Is Depression a Neurodevelopmental, Cognitive Disorder? Focus on Prefronto-Thalamic Circuits

Depression is one of the most burdensome psychiatric disorders, affecting hundreds of millions of people worldwide. The disease is characterized not only by severe emotional and affective impairments, but also by disturbed vegetative and cognitive functions. Although many candidate mechanisms have been proposed to cause the disease, the pathophysiology of cognitive impairments in depression remains unclear. In this article, we aim to assess the link between cognitive alterations in depression and possible developmental changes in neuronal circuit wiring during critical periods of susceptibility. We review the existing literature and propose a role of serotonin signaling during development in shaping the functional states of prefrontal neuronal circuits and prefronto-thalamic loops. We discuss how early life insults affecting the serotonergic system could be important in the alterations of these local and long-range circuits, thus favoring the emergence of neurodevelopmental disorders, such as depression.

Major depressive disorder: hypothesis, mechanism, prevention and treatment

Worldwide, the incidence of major depressive disorder (MDD) is increasing annually, resulting in greater economic and social burdens. Moreover, the pathological mechanisms of MDD and the mechanisms underlying the effects of pharmacological treatments for MDD are complex and unclear, and additional diagnostic and therapeutic strategies for MDD still are needed. The currently widely accepted theories of MDD pathogenesis include the neurotransmitter and receptor hypothesis, hypothalamic-pituitary-adrenal (HPA) axis hypothesis, cytokine hypothesis, neuroplasticity hypothesis and systemic influence hypothesis, but these hypothesis cannot completely explain the pathological mechanism of MDD. Even it is still hard to adopt only one hypothesis to completely reveal the pathogenesis of MDD, thus in recent years, great progress has been made in elucidating the roles of multiple organ interactions in the pathogenesis MDD and identifying novel therapeutic approaches and multitarget modulatory strategies, further revealing the disease features of MDD. Furthermore, some newly discovered potential pharmacological targets and newly studied antidepressants have attracted widespread attention, some reagents have even been approved for clinical treatment and some novel therapeutic methods such as phototherapy and acupuncture have been discovered to have effective improvement for the depressive symptoms. In this work, we comprehensively summarize the latest research on the pathogenesis and diagnosis of MDD, preventive approaches and therapeutic medicines, as well as the related clinical trials.

Astrocytic CREB in nucleus accumbens promotes susceptibility to chronic stress

Background

Increasing evidence implicates astrocytes in stress and depression in both rodent models and human Major Depressive Disorder (MDD). Despite this, little is known about the transcriptional responses to stress of astrocytes within the nucleus accumbens (NAc), a key brain reward region, and their influence on behavioral outcomes.

Methods

We used whole cell sorting, RNA-sequencing, and bioinformatic analyses to investigate the NAc astrocyte transcriptome in male mice in response to chronic social defeat stress (CSDS). Immunohistochemistry was used to determine stress-induced changes in astrocytic CREB within the NAc. Finally, astrocytic regulation of depression-like behavior was investigated using viral-mediated manipulation of CREB in combination with CSDS.

Results

We found a robust transcriptional response in NAc astrocytes to CSDS in stressed mice, with changes seen in both stress-susceptible and stress-resilient animals. Bioinformatic analysis revealed CREB, a transcription factor widely studied in neurons, as one of the top-predicted upstream regulators of the NAc astrocyte transcriptome, with opposite activation states seen in resilient versus susceptible mice. This bioinformatic result was confirmed at the protein level with immunohistochemistry. Viral overexpression of CREB selectively in NAc astrocytes promoted susceptibility to chronic stress.

Conclusions

Together, our data demonstrate that the astrocyte transcriptome responds robustly to CSDS and, for the first time, that transcriptional regulation in astrocytes contributes to depressive-like behaviors. A better understanding of transcriptional regulation in astrocytes may reveal unknown molecular mechanisms underlying neuropsychiatric disorders.

Homeostasis to Allostasis: Prefrontal Astrocyte Roles in Cognitive Flexibility and Stress Biology

In the intricate landscape of neurophysiology, astrocytes have been traditionally cast as homeostatic cells; however, their mechanistic involvement in allostasis-particularly how they modulate the adaptive response to stress and its accumulative impact that disrupts cognitive functions and precipitates psychiatric disorders-is now starting to be unraveled. Here, we address the gap by positing astrocytes as crucial allostatic players whose molecular adaptations underlie cognitive flexibility in stress-related neuropsychiatric conditions. We review how astrocytes, responding to stress mediators such as glucocorticoid and epinephrine/norepinephrine, undergo morphological and functional transformations that parallel the maladaptive changes. Our synthesis of recent findings reveals that these glial changes, especially in the metabolically demanding prefrontal cortex, may underlie some of the neuropsychiatric mechanisms characterized by the disruption of energy metabolism and astrocytic networks, compromised glutamate clearance, and diminished synaptic support. We argue that astrocytes extend beyond their homeostatic role, actively participating in the brain's allostatic response, especially by modulating energy substrates critical for cognitive functions.

Astroglial Connexin 43-Mediated Gap Junctions and Hemichannels: Potential Antidepressant Mechanisms and the Link to Neuroinflammation

Major depression disorder (MDD) is a neuropsychiatric disorder associated with a high suicide rate and a higher disability rate than any other disease. Evidence suggests that the pathological mechanism of MDD is related to astrocyte dysfunction. Depression is mainly associated with the expression of connexin 43 (Cx43) and the function of Cx43-mediated gap junctions and hemichannels in astrocytes. Moreover, neuroinflammation has been a hotspot in research on the pathology of depression, and Cx43-mediated functions are thought to be involved in neuroinflammation-related depression. However, the specific mechanism of Cx43-mediated functions in neuroinflammation-related depression pathology remains unclear. Therefore, this review summarizes and discusses Cx43 expression, the role of gap junction intercellular communication, and its relationship with neuroinflammation in depression. This review also focuses on the effects of antidepressant drugs (e.g., monoamine antidepressants, psychotropic drugs, and N-methyl-D-aspartate receptor antagonists) on Cx43-mediated function and provides evidence for Cx43 as a novel target for the treatment of MDD. The pathogenesis of MDD is related to astrocyte dysfunction, with reduced Cx43 expression, GJ dysfunction, decreased GJIC and reduced BDNF expression in the depressed brain. The effect of Cx43 on neuroinflammation-related depression involving inflammatory cytokines, glutamate excitotoxicity, and HPA axis dysregulation. Antidepressant drugs targeting Cx43 can effectively relieve depressive symptoms.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Astrocytes in Post-Stroke Depression: Roles in Inflammation, Neurotransmission, and Neurotrophin Signaling

Post-stroke depression (PSD) is a frequent and disabling complication of stroke that affects up to one-third of stroke survivors. The pathophysiology of PSD involves multiple mechanisms, including neurochemical, neuroinflammatory, neurotrophic, and neuroplastic changes. Astrocytes are a type of glial cell that is plentiful and adaptable in the central nervous system. They play key roles in various mechanisms by modulating neurotransmission, inflammation, neurogenesis, and synaptic plasticity. This review summarizes the latest evidence of astrocyte involvement in PSD from human and animal studies, focusing on the alterations of astrocyte markers and functions in relation to monoamine neurotransmitters, inflammatory cytokines, brain-derived neurotrophic factor, and glutamate excitotoxicity. We also discuss the potential therapeutic implications of targeting astrocytes for PSD prevention and treatment. Astrocytes could be new candidates for antidepressant medications and other interventions that aim to restore astrocyte homeostasis and function in PSD. Astrocytes could be new candidates for antidepressant medications and other interventions that aim to restore astrocyte homeostasis and function in PSD.

Electroacupuncture prevents astrocyte atrophy to alleviate depression

Astrocyte atrophy is the main histopathological hallmark of major depressive disorder (MDD) in humans and in animal models of depression. Here we show that electroacupuncture prevents astrocyte atrophy in the prefrontal cortex and alleviates depressive-like behaviour in mice subjected to chronic unpredictable mild stress (CUMS). Treatment of mice with CUMS induced depressive-like phenotypes as confirmed by sucrose preference test, tail suspension test, and forced swimming test. These behavioural changes were paralleled with morphological atrophy of astrocytes in the prefrontal cortex, revealed by analysis of 3D reconstructions of confocal Z-stack images of mCherry expressing astrocytes. This morphological atrophy was accompanied by a decrease in the expression of cytoskeletal linker Ezrin, associated with formation of astrocytic leaflets, which form astroglial synaptic cradle. Electroacupuncture at the acupoint ST36, as well as treatment with anti-depressant fluoxetine, prevented depressive-like behaviours, astrocytic atrophy, and down-regulation of astrocytic ezrin. In conclusion, our data further strengthen the notion of a primary role of astrocytic atrophy in depression and reveal astrocytes as cellular target for electroacupuncture in treatment of depressive disorders.

Genesis of a functional astrocyte syncytium in the developing mouse hippocampus

Astrocytes are increasingly shown to operate as an isopotential syncytium in brain function. Protoplasmic astrocytes acquire this ability to functionally go beyond the single-cell level by evolving into a spongiform morphology, cytoplasmically connecting into a syncytium, and expressing a high density of K+ conductance. However, none of these cellular/functional features exist in neonatal newborn astrocytes, which imposes a basic question of when a functional syncytium evolves in the developing brain. Our results show that the spongiform morphology of individual astrocytes and their spatial organization all reach stationary levels by postnatal day (P) 15 in the hippocampal CA1 region. Functionally, astrocytes begin to uniformly express a mature level of passive K+ conductance by P11. We next used syncytial isopotentiality measurement to monitor the maturation of the astrocyte syncytium. In uncoupled P1 astrocytes, the substitution of endogenous K+ by a Na+ -electrode solution ([Na+ ]p ) resulted in the total elimination of the physiological membrane potential (VM ), and outward K+ conductance as predicted by the Goldman-Hodgkin-Katz (GHK) equation. As more astrocytes are coupled to each other through gap junctions during development, the [Na+ ]p -induced loss of physiological VM and the outward K+ conductance is progressively compensated by the neighboring astrocytes. By P15, a stably established syncytial isopotentiality (-73 mV), and a fully compensated outward K+ conductance appeared in all [Na+ ]p -recorded astrocytes. Thus, in view of the developmental timeframe wherein a singular syncytium is anatomically and functionally established for intra-syncytium K+ equilibration, an astrocyte syncytium becomes fully operational at P15 in the mouse hippocampus.

Astroglia support, regulate and reinforce brain barriers

Nervous system is segregated from the body by the complex system of barriers. The CNS is protected by (i) the blood-brain and blood-spinal cord barrier between the intracerebral and intraspinal blood vessels and the brain parenchyma; (ii) the arachnoid blood-cerebrospinal fluid barrier; (iii) the blood-cerebrospinal barrier of circumventricular organs made by tanycytes and (iv) the choroid plexus blood-CSF barrier formed by choroid ependymocytes. In the peripheral nervous system the nerve-blood barrier is secured by tight junctions between specialised glial cells known as perineural cells. In the CNS astroglia contribute to all barriers through the glia limitans, which represent the parenchymal portion of the barrier system. Astroglia through secretion of various paracrine factors regulate the permeability of endothelial vascular barrier; in pathology damage or asthenia of astrocytes may compromise brain barriers integrity.

Astrocytes in the pathophysiology of neuroinfection

Key homeostasis providing cells in the central nervous system (CNS) are astrocytes, which belong to the class of cells known as atroglia, a highly heterogeneous type of neuroglia and a prominent element of the brain defence. Diseases evolve due to altered homeostatic state, associated with pathology-induced astroglia remodelling represented by reactive astrocytes, astroglial atrophy and astrodegeneration. These features are hallmarks of most infectious insults, mediated by bacteria, protozoa and viruses; they are also prominent in the systemic infection. The COVID-19 pandemic revived the focus into neurotropic viruses such as SARS-CoV2 (Coronaviridae) but also the Flaviviridae viruses including tick-borne encephalitis (TBEV) and Zika virus (ZIKV) causing the epidemic in South America prior to COVID-19. Astrocytes provide a key response to neurotropic infections in the CNS. Astrocytes form a parenchymal part of the blood-brain barrier, the site of virus entry into the CNS. Astrocytes exhibit aerobic glycolysis, a form of metabolism characteristic of highly morphologically plastic cells, like cancer cells, hence a suitable milieu for multiplication of infectious agent, including viral particles. However, why the protection afforded by astrocytes fails in some circumstances is an open question to be studied in the future.